I've not used CocosStudio yet, but Spine was developed purely for rich animation control, and can be used for everything from characters to GUI. It has many extensive animation features.
CocosStudio (from what I've read), uses action sequences to move things around a scene, so you should be able to use Spine and CocosStudio together?
Keep in mind Spine is not as fast as native cocos2d actions, so if you need to have a large number of sprites on scene, then Spine might not be a good choice.
Also, the Spine editor costs a base set amount, but if your app makes over another amount then you're required to get a pro license. However, the profits of apps are royalty free as corytrese said.